In the field of vehicles which are operated by combustion engines, there is a general demand for low emissions of regulated substances in the exhaust gases from the engine. In the case of a diesel engine (CI engine), these substances are primarily in the form of nitrogen oxide compounds (NOx), hydrocarbon compounds (HC), carbon monoxide (CO) and particulate matter.
Today's diesel engines can be operated with a relatively lean exhaust gas, i.e. with a relatively high concentration of oxygen. A lean exhaust gas is one which has an air/fuel ratio λ which is higher than 1. A rich exhaust gas is one which has an air/fuel ratio A which is less than 1.
In contrast to a conventional gasoline engine (SI engine) which is equipped with a so-called three-way catalyst for removing the major part of the nitrogen oxide compounds (NOx), hydrocarbon compounds (HC) and carbon monoxide (CO) from the exhaust gases, a modern diesel engine normally uses a particulate matter filter (also known as a “diesel particulate filter”, or DPF) for trapping particulate matter in the exhaust gas. Such a filter is suitably also provided with an oxidizing catalyst for oxidizing, i.e. removing, hydrocarbon compounds (HC) and carbon monoxide (CO) in the exhaust gas. However, in order to remove nitrogen oxide compounds (NOx) from the exhaust gas, a so-called lean LNT (or LNT) is normally used. In this regard, it is previously known to use an LNT and a particulate matter filter with a diesel engine, either as two separate units or as one single integrated unit.
In an LNT, the NOx compounds in the exhaust gas will be adsorbed by means of a NOx storing compound such as barium. It should be noted that the LNT can only adsorb a certain amount of NOx compounds. This means that it will eventually be “filled”, i.e. it will reach a limit for the adsorption process. In this situation, the LNT must be regenerated, which means that it must be brought to desorb, i.e. release, the accumulated NOx compounds. Such a “denitration” process is normally carried out by making the exhaust gas mixture relatively rich, i.e. with a surplus of fuel compared to the amount of oxygen that is available for the combustion, during a certain time period. This can be achieved by means of the engine being operated with a comparatively rich air/fuel mixture during a short time period, e.g. a few seconds. In this manner, the LNT will be “emptied” so that it thereafter can adsorb NOx compounds during a certain time period which lasts until a new regeneration becomes necessary.
A particular problem with a LNT is that during operation, it will be exposed to sulfur which originates from the engine fuel or oil and is fed through the LNT via the exhaust gas. The sulfur in the exhaust gas results in poisoning of the LNT which causes ageing of the LNT and also reduces the number of available NOx storage sites in the LNT, so that the oxidation capacity of the LNT is reduced. Gradually, the LNT will be deteriorated until it reaches a stage where sulfur regeneration of the LNT (also known as “de-sulfurisation” or “deSOx”) becomes necessary, in order to purge the LNT from adsorbed sulfur compounds.
In order to carry out a de-sulfurisation process, the engine is operated so as to provide a relatively rich exhaust gas mixture, i.e. having a surplus of fuel compared to the amount of oxygen that is available for the combustion, during a certain time period. Also, the de-sulfurisation process demands an increased exhaust gas temperature, more precisely an exhaust gas temperature which is at least approximately 600-650° C. This raised temperature is normally provided by means of so-called post-injections of fuel, i.e. injections of fuel during a late stage of the combustion procedure. Such post-injections generate a relatively high amount of heat, but only a relatively low amount of torque in the engine. By providing a rich exhaust gas mixture and an increased exhaust gas temperature during a certain time period, sulfur compounds will be emitted from the LNT.
The conversion efficiency of the LNT benefits from frequent de-sulfurisation actions, due to the fact that irreversible sulfur poisoning can be avoided in this way. In practical applications, the de-sulfurisation frequency is limited for fuel consumption reasons. In normal applications, de-sulfurisation of the LNT are carried out with an interval of approximately 1000-100000 km, depending mainly on the quality of the fuel being used, i.e. depending on the sulfur concentration in the fuel.
An unwanted side-effect of the de-sulfurisation process is that there is a risk for emissions of H2S, if the time period with a rich exhaust gas mixture becomes too long. Emissions of H2S causes a bad smell from the exhaust gas. For this reason, said time period must be limited so as to avoid H2S emissions.
Furthermore, as regards the above-mentioned particulate matter filter, it can be noted that such a filter is used for the removal of soot particles in the exhaust flow. To avoid a high back pressure and excessively high temperatures during soot burnout, the particulate matter filter must be regenerated on a regular basis. Such a regeneration process demands a relatively high temperature, at least approximately 580° C. Also, this regeneration process requires a lean exhaust gas mixture to provide a surplus of oxygen, which in turn is necessary for the combustion of the accumulated particulate matter in the filter.
However, the regeneration of a particulate matter filter is sensitive to the oxygen content in the exhaust gas, since the combination of high soot loading and excess oxygen during regeneration may result in a very high temperature in the particulate matter filter. This may in turn cause irreversible damage to the particle filter, more precisely in the form of substrate cracking or melting.
Both the above-mentioned regeneration actions require high exhaust temperatures, which inevitably result in increased fuel consumption. Consequently, a general problem in the field of exhaust gas purification is to carry out the above-mentioned sulfur regeneration processes of a LNT and regeneration of a particulate matter filter in an optimum manner, without unnecessarily increasing the fuel consumption and without the above-mentioned problems and disadvantages.
Regarding prior art, it can be noted that the patent document US 2003/0213235 teaches an apparatus for purifying exhaust gas in an internal combustion engine. In particular, said document teaches a particulate filter and a NOx trap, wherein regeneration of the particulate filter, and NOx regeneration and de-sulfurisation of the NOx trap are carried out.